(i) Field of the Invention
The present invention relates to an optical recording medium on which recording is carried out by the irradiation of light in near infrared region.
(ii) Description of the Prior Art
A heat-sensitive recording system is a direct recording system which does not require any development and fixing, and therefore it is excellent in operation and maintenance. For this reason, the heat-sensitive recording system is widely utilized in facsimiles, printers and the like.
In this system, however, the recording is thermally made by bringing a thermal head or an exothermic IC pen into direct contact with a heat-sensitive recording paper, and hence melted colored substances adhere to the thermal head or the exothermic IC pen, so that troubles such as dregs adhesion and sticking take place, which brings about record obstruction and impairs record quality inconveniently.
In particular, when a line is depicted continuously in a recording direction as in the case of a plotter printer, it is impossible to avoid the trouble of the dregs adhesion.
Furthermore, when the recording is made by the thermal head, it is difficult to more heighten an image resolution of 8 dots/mm which is now employed
Thus, as techniques by which troubles such as the dregs adhesion and the sticking are solved and by which the resolution is more improved, some non-contact recording systems using light have been suggested.
Japanese Patent Laid-open Publication No. 209594/1983 discloses an optical recording medium prepared by laminating at least one set of a near infrared absorbent layer having an absorption wave length in a near infrared region of 0.8 to 2 .mu.m and a heat-sensitive color-developing material layer onto a substrate, and Japanese Patent Laid-open Publication No. 94494/1983 discloses a recording medium prepared by superposing, on a base material, a layer containing one or more kinds of heat-sensitive materials and a layer containing one or more kinds of near infrared absorbents comprising compounds having a maximum absorption wave length in near infrared rays of 0.7 to 3 .mu.m.
These publications disclose the procedure of laminating or superposing the near infrared absorbent and the heat-sensitive color-developing material on the substrate or the base material That is, the near infrared absorbent is mixed with the heat-sensitive color-developing material and the resulting mixture is then applied onto the substrate or the base material, or alternatively the heat-sensitive color-developing material is first applied on the substrate or the base material, and the near infrared absorbent is then applied on the heat-sensitive color-developing material layer.
In the above-mentioned publications, there are disclosed dyestuffs such as cyanine dyestuffs, thiol nickel complexes and squalilium as the near infrared absorbent.
In addition, as enumerated in "Near Infrared Absorption Dyestuffs", Chemical Industry, Vol. 43, May 1986, other dyestuffs are known which are, for example, nitroso compounds and their metal complexes, polymethine dyestuffs (cyanine dyestuffs), complexes of thiols and cobalt or palladium, phthalocyanine dyestuffs, triallylmethane dyestuffs, immonium dyestuffs, diimmonium dyestuffs and naphthoquinone dyestuffs.
When these conventional near infrared absorbent and heat-sensitive color-developing material are mixed and then applied on the substrate as disclosed in the above-mentioned publication, a desensitization phenomenon appears and a color development performance deteriorates. Furthermore, when the heat-sensitive color-developing material is first applied on the substrate, and the near infrared absorbent is then applied on this material layer, the deterioration in a ground color occurs inconveniently. In addition, all the conventional near infrared absorbents have relatively strong and wide absorption spectra in a visual region, and they are also intensively colored. Therefore, it is difficult to obtain the optical recording medium having a white ground color, which obstructs putting the optical recording medium into practice.